The invention herein relates to magnetron energizing circuits for providing pulsed d.c. power to a magnetron from a relatively low frequency low voltage a.c. power source.
U.S. Pat. No. 3,396,342 and 3,902,099 are hereby incorporated herein by reference for the disclosures thereof.
The first of these patents discloses the theory of operation of the basic circuit utilizing the principles which are applied in this invention. The second of these patents discloses circuitry to enable two relatively small transformers to be used for achieving a higher power output by connecting them in parallel to operate suitable circuitry energizing a single magnetron.
The problem with the circuitry of the second patent has been that the peak currents which result are twice those which would occur through the use of a single transformer and the related circuit as taught therein. In each case the secondaries of the transformers were connected in parallel, resulting in their being phased in the same direction relative to the charging of the capacitors they served. In the invention herein, by connecting the secondaries in opposite directions with respect to the charging of the condensers they serve there has been achieved a distinct advantage which consists of a much lower peak current.
Specifically, the circuit as connected in accordance with the invention provides practically double power because of full wave pulsing but the peak currents are the same as though only one transformer were used. Further, the circuit can be converted from a full wave voltage doubler circuit into a half wave voltage doubler circuit merely by throwing a switch which cuts out one of the transformers and half of the circuit. This enables the achievement of low power, as for example, when it is desired to defrost food or cook at a low heat.
One of the most important aspects of the invention lies in the provision of a half wave voltage doubler magnetron energizing circuit in which the transformer expense is substantially decreased by using one or two economical transformers instead of one very expensive transformer. The circuit which uses two transformers then has the additional advantage mentioned above, namely, it can be switched to a half wave voltage doubler circuit; however, the switching is done at a very low voltage part of the circuit.
Looking at the conventional full wave voltage doubler circuit, the transformer secondary is at high potential at both terminal ends. Building a transformer for handling thousands of volts in the secondary winding calls for very expensive procedures. The problem is a practical one because the start of the coil of wire that forms the secondary is very close to the steel core if the coil window fits the core closely. This can result in arcing unless very expensive insulation is used, and even then there is no assurance that arcing will not occur where peaks of voltage are inordinately high during switching.
On this account, notwithstanding the benefits of full wave doubler circuits--i.e., greater power with low peak currents if the technique of U.S. Pat. No. 3,396,342 is used, these circuits are not as popular as the half wave voltage doubler circuit (FIG. 5 of U.S. Pat. No. 3,396,342). The full wave rectifier circuit (FIG. 2 of U.S. Pat. No. 3,396,342) has the same disadvantage as the full wave doubler circuit in requiring a fully insulated secondary winding.
It should be mentioned that the most usual method of building a transformer for use in a circuit which requires a fully insulated secondary winding is to make the coil window substantially larger than the core cross section so that the corner of the coil where greatest voltage stress occurs is spaced from the steel core.
In the case of the normal half wave voltage doubler circuit, one end of the transformer is rounded. This makes the construction of the transformer very economical because there is no difficulty in achieving sufficient insulation for the start of the coil at the corner.
The invention preferably utilizes two of these transformers, there being a wide variety of them commercially available and easily adaptable to the circuit of the invention using almost any type of magnetron or power requirements. It may seem anomolous, but the fact is that at the present time the cost of the two transformers with grounded end terminals on their secondaries may be less than the cost of a transformer with the secondary insulated for use with the same type of circuit. This presumes availability of the latter type of transformer which at the present time is questionable. Rectifiers required for the circuit of the invention are not a factor of added expense since they are commonly available at low cost.
The possibility exists of switching the known full wave voltage doubler to have it operated at half power as a half wave voltage doubler, but this is not practical because the switching must be done at high voltage. The invention provides low voltage switching with consequent elimination of the difficulty and expense of high voltage switching means.
Instead of two transformers with grounded secondaries the circuit can be used with one transformer having two secondaries that are grounded at respective terminal ends. This latter transformer can be built using production techniques with resulting economy so that the benefits of a full wave voltage doubler circuit is more readily available for low cost microwave ovens. Use of a transformer with only a single primary winding does not permit low voltage switching from full wave voltage doubler operation to half wave voltage doubler operation.